Chapter 5: Data Communication Fundamentals Notes

Data: Entities that convey meaning or information.
Transmission: The communication of data across a computer network by the propagation and processing of signals.
Signal: Electric or electromagnetic representation of data.
Signaling: The physical propagation of the signal along a communication medium.
The way in which the electromagnetic signals are encoded to convey data determines the efficiency and reliability of the transmission.

Analog data: Continuous values on some interval; Voice and video; Data collected by sensors, such as temperature and pressure.
Digital data: Discrete values; Text, integers, binary data.

Analog signal: Continuously varying electromagnetic wave that may be transmitted over both guided and unguided media.
Digital signal: Sequence of voltage pulses.
Advantages of digital signaling: Generally cheaper than analog signaling; Less susceptible to noise interference.
Disadvantages/limitations: Suffer more from attenuation than analog signals; Cannot be used on optical fiber or wireless media.

Analog data → Analog signal: Transmitter technologies use modulation techniques that enable sound and/or video waveforms to be conveyed as electromagnetic waveforms over wires or airwaves.
Modem (modulator/demodulator): Converts binary voltage pulses by modulating a carrier frequency; Demodulates the signal to recover the original data at the other end.
Codec (coder/decoder): Takes an analog signal and approximates that signal by a bit stream; At the other end of a line the bit stream is used to reconstruct the analog data.
Digital data → Analog signal: Converts digital data to an analog form for transmission.

Digital data, digital signal: The equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog equipment.
Analog data, digital signal: Conversion of analog data to digital form permits the use of modern digital transmission and switching equipment.
Digital data, analog signal: Some transmission media, such as optical fiber and satellite, only propagate analog signals.
Analog data, analog signal: Analog data are easily converted to an analog signal.

Analog transmission: Only transmits analog signals, without regard for data content.
Attenuation: Overcome with amplifiers.
Signal processing: Signal is not evaluated or regenerated.
Digital transmission: Transmits analog or digital signals; Uses repeaters rather than amplifiers.
Switching equipment: Evaluates and regenerates signal.

Cost: Large-scale and very large-scale integration has caused continuing drop in cost.
Data Integrity: Effect of noise and other impairments is reduced.
Capacity Utilization: High capacity is more easily and cheaply achieved with time division rather than frequency division.
Security and Privacy: Encryption possible.
Integration: All signals (voice, video, image, data) treated the same.

Analog or digital data must be converted into a signal for purposes of transmission.
Encoding scheme: The mapping from binary digits to signal elements.
Carrier signal: A continuous constant frequency signal used as the basis for analog encoding.
Modulation: The conversion of digital signals to analog form.
Demodulation: The conversion of analog data signals back to digital form.

Figure 5.2 illustrates modulation concepts (modulation of analog signals to carry digital data).

Modems remain widely used pieces of communications gear.
Function: Modulates an analog carrier wave to encode digital information; Demodulates signals to decode transmitted information.
Uses: Direct broadcast satellite, Wi-Fi, and mobile phones use modems to communicate.
Popular types: Voice-grade Cable, ADSL.

Cable modems (Figure 5.3) and ADSL modems (Figure 5.4) illustrate real-world implementations.
Fiber to the Home (Figure 5.5) shows fiber-based access architectures.

Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components and transmitted on a voice-grade telephone line.
Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band.
A higher frequency may be needed for effective transmission.
Analog-to-analog modulation permits frequency-division multiplexing.

Definition: Block of bits transmitted in a steady stream without start and stop codes.
Clocks synchronization: Clocks of transmitter and receiver must be synchronized.
- Approach 1: Separate clock line between transmitter and receiver (works well over short distances).
- Approach 2: Embed the clocking information in the data signal.
Frame: Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern; Data plus preamble, postamble, and control information are called a frame.
Data link control: Involves a data link control procedure which automatically detects transmission error and causes a frame in error to be retransmitted.

All transmission media have potential for introduction of errors.
Data link layer protocols must provide a method for controlling errors.
Error control process has two components:
- Error detection: Redundancy introduced so that the occurrence of an error will be detected.
- Error correction: Receiver and transmitter cooperate to retransmit frames that were in error.

Parity bit: Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity).
Limitations: Good for detecting single-bit errors only.
Noise impulses: Are often long enough to destroy more than one bit.
Effectiveness depends on: The total number of bits corrupted by noise impulses and the parity convention used.
Parity formulas:
- Even parity condition: $\sum<em>i b</em>i \equiv 0 \pmod{2}$
- Odd parity condition: $\sum<em>i b</em>i \equiv 1 \pmod{2}$

Table summarizes the various combinations of data types and signal types (no detailed content provided in the transcript).

Key topics covered in Chapter 5: Data and digital data concepts; encoding techniques; analog encoding of analog information; digital encoding of analog data; digital encoding of digital data; analog encoding of analog information; asynchronous vs synchronous transmission; error detection and error control; parity checks; cyclic redundancy check (CRC).
Emphasizes the tradeoffs between analog and digital traditions, the role of modulation and encoding, the function of modems, and the mechanisms for ensuring reliable transmission through synchronization and error control.
Practical implications include design choices for cost, integration, capacity, security, and compatibility across media such as copper, cable, fiber, and wireless.